Vol. IV. May, 1903. No. 6 



BIOLOGICAL BULLETIN. 



THE FORMATION OF THE NOTOCHORD IN THE 



AMPHIBIA. 



HELEN DEAN KING. 



A study of the mode of development of the notochord in the 

 common toad, Bnfo Icntiginosns, and of the frog, Rana palus- 

 tris, has brought to light certain points that have a bearing on 

 the formation of the same structure in related groups. A vast 

 amount of work has already been done along this line, yet a 

 wide difference of opinion exists among embryologists regarding 

 the origin of the notochord in the Amphibia. It is hoped that 

 the results recorded in the present paper may help to clear up 

 this question. 



The material used was fixed in corrosive-acetic (5 glacial 

 acetic acid), and the sections were stained on the slide with a 

 mixture of borax-carmine and Lyon's blue as described in a pre- 

 vious paper (King, 1 1). This stain gives particularly good results 

 when it is used on freshly preserved material, as then all of the 

 nuclei become dark red, the ectoderm and mesoderm appear dark 

 blue, while the yolk cells take but a pale blue tint and, therefore, 

 are easily distinguished from the other cells. This sharp defini- 

 tion of the tissues was of great assistance, particularly in the study 

 of the sections of Bitfo. All of the drawings given in the present 

 paper were outlined with the aid of a camera lucida. 



BUFO LENTIGINOSUS. 



When the circular blastopore is closing in, the mesoderm, 

 already differentiated from the other tissues, forms a continuous 

 sheet of small, angular, slightly pigmented cells across the dorsal 

 wall of the archenteron. In the middle and also in the anterior 

 part of the embryo, the mesoderm is entirely separated from the 

 ectoderm above as well as from the endoderm beneath it. In 

 the region just in front of the blastopore, the mesoderm is also 



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